Method for working with sorted arrays - use a
binary chop to determine the insertion location
for an object. If equal objects already exist in the
array, they will be located immediately before the
insertion position.

The comparator function takes two items as arguments,
the first is the item to be added, the
second is the item already in the
array. The function should return
NSOrderedAscending if the
item to be added is 'less than' the
item in the array, NSOrderedDescending
if it is greater, and NSOrderedSame if it is equal.